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Thursday, January 30, 2014

Today the Moon passed between SDO and the Sun from 1331-1556 UTC (8:31-1056 am ET). At 2.5 hours in length, it was the longest lunar transit so far in the SDO mission.

You can watch the transit using the browse data option on our website. Use the times 2014-01-30 00:00:00 and 2014-01-30 17:00:00 and your favorite wavelength to enjoy the show.

Here is an image from 2014-01-30 14:29:12 in the AIA 171 passband. If you watch the movie you will see the Sun move a little bit during the transit. This transit covers a lot of the solar disk and blocks the sunlight from SDO. The fine guidance systems on AIA and HMI can't work because they need to see the whole Sun to keep the images centered from exposure to exposure. Once the transit is over the fine guidance systems started back up, giving us steady images of Sun.

And, as the Moon left SDO's field of view, an M8 flare erupted in AR 11967.

Saturday, January 4, 2014

Today, January 4, at 1200 UTC (7 am EST), the Earth was at perihelion. This is when we were closest to the Sun in our orbit around the Sun. At perihelion, the center of the Earth was a little over 147 million km (about 91.4 million miles) from the center of the Sun. Why do we care?

The Earth orbits the Sun along an ellipse. That means sometimes we’re farther from the Sun and sometimes closer. The average distance of the Earth from the Sun is about 150 million km (about 93 million miles, one Astronomical Unit or AU). Perihelion is the starting time for that orbit. The distance of perihelion is also important. The perihelion distance of the Earth is about 1.67% closer than the average distance.

When the Earth is closer to the Sun it receives more sunlight and when it is further away it receives less sunlight. That means the timing and distance of perihelion can affect our climate. That doesn’t mean that being closer to the Sun automatically means warmer weather.

Our seasons are caused by the tilt of the earth’s rotation axis compared to our orbit around the Sun. In January the northern hemisphere of the Earth is tilted away from the Sun and the northern latitudes have winter. At the same time those living in southern latitudes experience summer. Because the tilt determines the seasons we mark our seasons by the solstices and equinoxes.

But the changing distance does affect our climate. It appears to make southern winters a little less frigid and northern summers a little milder. In the past the difference was more dramatic.

The timing of perihelion in the year changes slowly. Right now perihelion happens in northern winter, while 11,000 years ago it happened in northern summer. That changed the severity of the seasons.

If we wait even longer (several hundred thousand years) the shape of the Earth’s orbit also changes, with the eccentricity changing from nearly 0 (almost circular) to the current value of 0.0167 to 0.06. A circular orbit means the distance to the Sun does not change, while a more elliptical orbit means the amount of sunlight hitting the Earth changes even more during the year. Right now the amount of sunlight hitting the Earth at perihelion is about 6.5% greater than what hits the Earth at the furthest distance. At an eccentricity of 0.06 that would increase to 27%.

These are just two of the Milankovitch cycles that were developed to explain glaciations.

SDO instruments were built to allow for the changing distance and apparent size of the Sun during a year. But it’s nice to know that those changes do other things as well.